In a system where an untrusted provider gives a product or provides a service to an untrusted client on behalf of a third party, such as a trustee, the occurrence of a transaction may be verified synchronously or asynchronously. For example, confirmation of package delivery, coupon usage, and access privileges are common scenarios where both the client and provider may be unknown and/or untrusted by a third party (a trustee) who needs to verify a transaction. Where a trustee has a new or limited-trust relationship with the providing party, and it may sometimes be inconvenient, inefficient, or costly to synchronously communicate with the trustee during each transaction between a client and a provider.
Some existing systems use physical tokens or physical tickets to validate a transaction. For example, physical tokens may be used to operate a machine in a pet store. The pet store has tokens in the register that are sold to clients, who then put them in a machine that makes pet identification tags. The provider of the machine then redeems the tokens periodically (e.g., weekly or monthly) with the trusted pet store owner for cash. The pet store owner, who is the trustee, doesn't need to maintain or secure the provider's machine. The provider does not need to store money in her machine over long periods, which reduces the chances of theft. Since the tokens are valuable only to the pet store owner (trustee) or the provider, there is no incentive for outside parties to steal tokens from the machine. Because tokens are used, they can be tracked separately from general purchases of the pet store. The client purchasing tokens from the trustee, the client use of tokens to get the ID tags, and the eventual provider redemption of the tokens with the trustee are all asynchronous, meaning that they don't require all three parties' presence. Similar physical token and ticket systems are used for arcades, raffles, laundromats, subway admission systems, and slot machines to avoid fraud by untrusted employees acting as “providers” servicing machines and customers.
However, physical tokens may be counterfeited, the client may lose the tokens or may decide not to use them and therefore want a refund. On the other hand, the provider may counterfeit tokens and attempt to redeem the counterfeit tokens with the trustee.
Some existing systems utilize electronic currency, such as credit cards or gift cards. The use of credit cards or gift cards often requires synchronous communication with the trustee. For example, credit card transactions require that the provider electronically verifies that the funds will be reimbursed to a provider before products or services are released to a client.